Increased knowledge of the coordination chemistry of technetium (Tc) should significantly aid in the effort to develop tumor-specific radiodiagnostic agents containing this element. The goals of this research are to: 1) explore the chemistry of the lower oxidation states of Tc and 2) to synthesize a series of radioscintigraphic tumor-diagnostic agents containing 99mTc in the I to IV oxidation states. Stabilization of the lower technetium oxidation states is being attempted with pyridine and bipyridine ligands. The Tc(I) and Tc(II) oxidation states are expected to coordinate preferentially to nitrogen rather than oxygen ligands. Potential tumor diagnostic agents will fall into two categories: 1) those in which Tc(I) or Tc(II) ions should bind directly to target sites, and 2) those containing Tc firmly coordinated in higher oxidation states by ligands which can be expected to direct the complex toward neoplasms. The first class of compounds will contain groups which can dissociate from the metal and so leave sites open for direct coordination. Chemical modification of the nonleaving groups should provide a means for varying the substitution rates and reduction potentials of these complexes so as to control their in vivo activity. A crystal structure of tr- O2(en)2Tc(V) plus has recently been obtained with Tc-O bond lengths of 1.76 Angstroms and Tc-N distances of 2.15 Angstroms. Crystals containing 4-aminopyridine, pyridine and bipyridine are presently under investigation by x-ray crystallography.